Periodic cicadas are cicadas which spend all but a very few weeks of their long lives silent and immobile underground. Then almost simultaneously millions of nymphs emerge from holes in the earth and, within hours, become gregarious flying adults[1]. Within a short time however, the adults of that brood are all dead, leaving behind eggs that will hatch to become a new underground generation, one that will emerge again in. The "emergence" is coordinated over extended periods of time - 7, 13, 17 year cycles are thought to reflect a strategy of avoiding other species on different cycles to avoid mating confusion and cross breeding [citation needed]. The numbers as prime numbers mean that for example being on a seven and a 13 year cycle two species would only emerge together once in every 200+ years. Wheres if they were on 2 and 4 year cycles they would meet far more frequently.